[speex-dev] Comments on New RTP Profile Document
Greg Herlein
gherlein at herlein.com
Sat Feb 22 12:12:11 PST 2003
The latest revision of the draft profile for use of Speex in RTP
is attached. We plan on submitting this - or a modified version
of this, based on immediate feedback - to the IETF on Monday for
consideration at the next meeting.
Major differences in this revision are:
- removed the discussion in the MIME section. It's a duplicate
of the SDP discussion anyway, and may or may not match the Ogg
MIME efforts, so it's best to eliminate it from here and let the
Ogg side handle that. All we want in this profile is how to use
it in RTP anyway. I may be angering some by this, but if we are
to get this to the IETF by Monday, then we can't wait to
integrate with the Ogg side now.
- added in ultrabandwidth, which was developed after the last
version was written.
- added references for the H.323 documents.
Comments are always welcome - especially if they come soon enough
that I can get them incorporated by Monday (actually, by Sunday
evening). By the way, those day/times are USA west coast Pacific
Daylight Time.
Greg
<p><p><p><p>
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Internet Engineering Task Force Greg Herlein
Internet Draft Jean-Marc Valin
draft-herlein-speex-rtp-profile-08 Simon Morlat
February, 2002
Expires: July, 2003
RTP Payload Format for the Speex Codec
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-Drafts
as reference material or to cite them other than as "work in
progress".
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
To view the list Internet-Draft Shadow Directories, see
http://www.ietf.org/shadow.html.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
Speex is an open-source, patent-free voice codec suitable for use in
Voice over IP (VoIP) type applications. The Speex codec supports two
modes of operation: narrowband at a nominal 8kHz sample rate,
wideband at a nominal 16kHz sample rate, and ultra-wideband at a
nominal 32kHz sample rate. Speex supports Voice Activity Detection
(VAD) and Variable Bit Rate (VBR). This document describes the
payload format for Speex generated bit streams within an RTP packet.
Also included here are the necessary details for the use
of Speex with the Session Description Protocol (SDP) [4] and a preliminary
method of using Speex within H.323 applications. Use of Speex with
MIME will be covered as part of the Ogg Vorbis MIME definitions and is
covered only minimally here.
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1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [5].
2. Overview of the Speex Codec
Speex is based on the CELP encoding technique with support for either
wideband (nominal 16kHz) or narrowband (nominal 8kHz) sampling. The
main characteristics can be summerized as follows:
o Free software/open-source, royalty-free
o Integration of wideband and narrowband in the same bit-stream
o Wide range of bit-rates available
o Dynamic bit-rate switching and variable bit-rate (VBR)
o Voice Activity Detection (VAD, integrated with VBR)
o Variable complexity
3. RTP payload format for Speex
Speex uses 20 ms frames and a variable sampling rate clock. The
RTP timestamp MUST be in units of 1/X of a second where X is the
sample rate used. Speex uses a nominal 8kHz sampling rate for
narrowband use, a nominal 16kHz sampling rate for wideband use, and
a nominal 32kHz sampling rate for ultra-wideband use.
The RTP payload for Speex has the format shown in Figure 1. No
additional header specific to this payload format is required.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header [2] |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| |
+ one or more frames of Speex |
| .... |p|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: RTP payload for Speex
The encoding and decoding algorithm can change the bit rate at any
20ms frame boundary but the bit rate change notification is provided
in-band with the bit stream. Each frame contains both
"mode" (narrowband or wideband) and "sub-mode" (bit-rate)
information in the bit stream. No out-of-band notification is
required for the decoder to process changes in the bit rate sent by
the encoder.
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Internet-Draft RTP Payload Format for the Speex Codec Nov 2002
For the purposes of packetizing the bit stream in RTP, it is only
necessary to consider the sequence of bits as output by the Speex
encoder, and present the same sequence to the decoder. The payload
format described here maintains this sequence.
An RTP packet MAY contain Speex frames of the same bit rate or of
varying bit rates, since the bit-rate for a frame is conveyed in
band with the signal.
It is RECOMMENDED that values of 8000 or 16000 be used for normal
internet telephony applications, though the sample rate is
supported at rates as low as 6000 Hz and as high as 32 kHz.
The RTP payload MUST be padded to provide an integer number of
octets as the payload length. These padding bits MUST be all zero.
This padding is only required for the last frame in the packet, and
only to ensure the packet contents ends on an octet boundary.
3.1 RTP Payload Type Codes
The RTP Audio-Visual Working Group will no longer issue static
payload type codes for RTP (beyond those already assigned).
Dynamic payload type codes MUST be negotiated 'out-of-band'
for the assignment of a dynamic payload type from the
range of 96-127. Examples of this are shown in the section
discussing the Session Description Protocol (SDP) below.
3.2 Multiple Speex frames in a RTP packet
By default only one Speex frame is permitted in a single RTP
packet. When operating with multiple frames per packet then the
end points MUST use out-of-band negotiation to determine the number
of frames per packet. See section 5 below for an example of how to
do this with SDP [4].
3.3 Computing the number of Speex frames
If using SDP [4] (see section 5 below for an example) this can be
done using the "ptime" variable to denote the packetization
interval (ie, how many milliseconds of audio is encoded in a single
RTP packet). Since Speex uses 20ms frames, ptime values of
multiples of 20 denote multiple Speex frames per packet. Values of
ptime in other than multiples of 20 SHOULD be ignored and SHOULD
use the default value of one instead.
4. MIME registration of Speex
Full definition of the MIME type for Speex will be part of the Ogg
Vorbis MIME type definition application.
MIME media type name: audio
MIME subtype: speex
Required parameters: to be included in the Ogg MIME specification.
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Internet-Draft RTP Payload Format for the Speex Codec Nov 2002
Optional parameters:
Encoding considerations:
Security Considerations:
See Section 6 of RFC 3047.
Interoperability considerations: none
Published specification:
Applications which use this media type:
Additional information: none
Person & email address to contact for further information:
Greg Herlein <gherlein at herlein.com>
Jean-Marc Valin <jean-marc.valin at hermes.usherb.ca>
Intended usage: COMMON
Author/Change controller:
Author: Greg Herlein <gherlein at herlein.com>
Change controller: Greg Herlein <gherlein at herlein.com>
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Internet-Draft RTP Payload Format for the Speex Codec Nov 2002
5. SDP usage of Speex
When conveying information by SDP [4], the encoding name SHALL be
"speex". An example of the media representation in SDP for
offering a single channel of Speex at 8000 samples per second might
be:
m=audio 8088 RTP/AVP 97
a=rtpmap:97 speex/8000
Note that the RTP payload type code of 97 is defined in this media
definition to be 'mapped' to the speex codec at an 8kHz sampling
frequency using the 'a=rtpmap' line. Any number from 96 to 127
could have been chosen (the allowed range for dynamic types). The
value of the sampling frequency is typically 8000 for narrow band
operation, 16000 for wide band operation, and 32000 for ultra-wide
band operation.
If for some reason the offerer has bandwith limitations, he may use
the "b=" header, as explained in SDP [4]. The following example
illustrates the case where the offerer cannot receive more than
10 kbit/s.
m=audio 8088 RTP/AVP 97
b=AS:10
a=rtmap:97 speex/8000
In this case, if the remote part agrees, it should configure its
speex encoder so that it does not use modes that produce more than
10 kbit/s. Note that the "b=" constraint also applies on all
payload types that may be proposed in the media line ("m=").
An other way to make recommendations to the remote speex encoder
is to use its specific parameters via the a=fmtp: directive. The
following parameters are defined for use in this way:
ptime: duration of each packet in milliseconds.
sr: actual sample rate in Hz.
ebw: encoding bandwidth - either 'narrow' or 'wide' or
'ultra' (corresponds to nominal 8000, 16000, and
32000 Hz sampling rates).
vbr: variable bit rate - either 'on' 'off' or 'vad'
(defaults to off). If on, variable bit rate is
enabled. If off, disabled. If set to 'vad' then
constant bit rate is used but silence will be encoded
with special short frames to indicate a lack of voice
for that period.
cng: comfort noise generation - either 'on' or 'off'. If
off then silence frames will be silent; if 'on' then
those frames will be filled with comfort noise.
mode: speex encoding mode. Can be {1,2,3,4,5,any}
defaults to 3.
penh: use of perceptual enhancement. 1 indicates
to the decoder that perceptual enhancement is recommended,
0 indicates that it is not. Defaults to no.
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Internet-Draft RTP Payload Format for the Speex Codec Nov 2002
Examples:
m=audio 8008 RTP/AVP 97
a=rtpmap:97 speex/8000
a=fmtp:97 mode=4
This examples illustrate an offerer that wishes to receive
a speex stream at 8000Hz, but only using speex mode 3.
The offerer may suggest to the remote decoder to activate
its perceptual enhancement filter like this:
m=audio 8088 RTP/AVP 97
a=rtmap:97 speex/8000
a=fmtp:97 penh=1
Several speex specific parameters can be given in a single
a=fmtp line provided that they are separated by a semi-colon:
a=fmtp:97 mode=any;penh=1
The offerer may indicate that it wishes to send variable bit rate
frames with comfort noise:
m=audio 8088 RTP/AVP 97
a=rtmap:97 speex/8000
a=fmtp:97 vbr=on;cng=on
The use of a particular packetization interval may be
suggested to the remote encoder using the ptime parameter:
m=audio 8008 RTP/AVP 97
a=rtpmap:97 speex/8000
a=ptime:40
Note that the ptime parameter applies to all payloads listed
in the media line and is not used as part of an a=fmtp directive.
Speex can encode frames of 20 ms. Values of ptime not multiple
of 20 ms are meaningless, so the receiver of such ptime values
SHOULD ignore them.
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Internet-Draft RTP Payload Format for the Speex Codec Nov 2002
6. ITU H.323/H.245 Use of Speex
Application is underway to make Speex a standard ITU codec.
However, until that is finalized, Speex MAY be used in H.323 [6] by
using a non-standard codec block definition in the H.245 [7] codec
capability negotiations.
6.1 NonStandardMessage format
For Speex use in H.245 [7] based systems, the fields in the
NonStandardMessage should be:
t35CountryCode = Hex: B5
t35Extension = Hex: 00
manufacturerCode = Hex: 0026
[Length of the Binary Sequence (8 bit number)]
[Binary Sequence consisting of an ASCII string, no NULL terminator]
The binary sequence is an ascii string merely for ease of use.
The string is not null terminated. The format of this string is
speex [optional variables]
The optional variables are identical to those used for the SDP
a=fmtp strings discussed in section 5 above. The string is built
to be all on one line, each key-value pair seperated by a
semi-colon. The optional variables MAY be ommited, which causes
the default values to be assumed. They are:
ebw=narrow;mode=3;vbr=off;cng=off;ptime=20;sr=8000;penh=no;
The fifth byte of the block is the length of the binary sequence.
NOTE: this method can result in the advertising of a large number
of Speex 'codecs' based on the number of variables possible. For
most VoIP applications, use of the defailt binary sequence of
'speex' is RECOMMENDED to be used in addition to all other options.
This maximizes the chances that two H.323 based applications that
support Speex can find a mutual codec.
6.2 RTP Payload Types
Dynamic payload type codes MUST be negotiated 'out-of-band'
for the assignment of a dynamic payload type from the
range of 96-127. H.323 applications MUST use the H.245
H2250LogicalChannelParameters encoding to accomplish this.
7. Security Considerations
RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification [2], and any appropriate RTP profile. This implies
that confidentiality of the media streams is achieved by encryption.
Because the data compression used with this payload format is applied
end-to-end, encryption may be performed after compression so there is
no conflict between the two operations.
A potential denial-of-service threat exists for data encodings using
compression techniques that have non-uniform receiver-end
computational load. The attacker can inject pathological datagrams
into the stream which are complex to decode and cause the receiver to
be overloaded. However, this encoding does not exhibit any
significant non-uniformity.
As with any IP-based protocol, in some circumstances a receiver may
be overloaded simply by the receipt of too many packets, either
desired or undesired. Network-layer authentication may be used to
discard packets from undesired sources, but the processing cost of
the authentication itself may be too high.
8. References
1. Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
2. Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP:
A Transport Protocol for real-time applications", RFC 1889,
January 1996. (Updated by a Work in Progress.)
3. Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.
4. Handley, M. and V. Jacobson, "SDP: Session Description Protocol",
RFC 2327, April 1998.
5. Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
6. ITU-T Recommendation H.323. "Packet-based Multimedia Communications
Systems," 1998.
7. ITU-T Recommendation H.245 (1998), "Control of communications
between Visual Telephone Systems and Terminal Equipment".
9. Acknowledgments
The authors would like to thank Equivalence Pty Ltd of Australia
for their assistance in attempting to standardize the use of Speex
in H.323 applications, and for implementing Speex in their open
source OpenH323 stack. The authors would also like to thank Brian
C. Wiles <brian at streamcomm.com> of StreamComm for his assistance in
developing the proposed standard for Speex use in H.323
applications.
10. Author's Address
Greg Herlein <gherlein at herlein.com>
2034 Filbert Street
San Francisco, CA
United States 94123
Jean-Marc Valin <jean-marc.valin at hermes.usherb.ca>
Department of electrical and computer engineering
University of Sherbrooke
2500 blvd Université
Sherbrooke, Quebec, Canada, J1K 2R1
Simon MORLAT <simon.morlat at linphone.org>
35, av de Vizille App 42
38000 GRENOBLE
FRANCE
Roger Hardiman <roger at freebsd.org>
49 Nettleton Road
Cheltenham
Gloucestershire
GL51 6NR
England
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10. Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Herlein, Valin, etc [Page 8]
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